The present invention relates generally to a semiconductor device, and, more particularly, to a CMOS image sensor capable of preventing a surface of a metal line therein from being damaged or contaminated, thereby improving performance of a semiconductor memory device.
As is well known, an image sensor is an apparatus generating image data by sensing a light beam reflected from an object. An exemplary image sensor fabricated by a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels, each unit pixel including a light sensing element and a plurality of transistors. The light sensing element, such as a photodiode, senses an incident light beam and generates photoelectric charges corresponding to the amount of the incident light beam received. The transistors perform switching operations to control transfer of the photoelectric charges.
FIGS. 1A to 1D are cross-sectional views showing sequential steps of fabricating a conventional CMOS image sensor. Here, reference numerals 100 and 150 represent a unit pixel area and a pad area, respectively. For convenience purposes, the distance between the unit pixel area 100 and the pad area 150 has been truncated by a split view.
Referring to FIG. 1A, a series of operations are carried out to provide a semiconductor structure that has a metal line 101 on an upper portion thereof, the metal line 101 extending from the unit pixel area 100 into the pad area 150. After forming an anti-reflection layer 102 of about 300 xc3x85 on the metal line 101, an oxide layer 103 and a nitride layer 104, together forming a passivation layer, are stacked on the entire structure extending from the unit pixel area 100 into the pad area 150.
Referring to FIG. 1B, the nitride layer 104, the oxide layer 103 and the anti-reflection layer 102 are selectively etched to expose a portion of the metal line 101 and create a pad open area 105.
Referring to FIG. 1C, a dyed photoresist 106 is coated on the entire structure. Then, an exposure and development operation is carried out to form a color filter 106 over a light sensing region of the CMOS image sensor. At this time, a portion of the dyed photoresist 106, i.e., the photoresist pattern 106b, covering the pad open portion 105, is removed during the develop operation. Then, a thermal treatment is performed at a temperature of 145xc2x0 C. to 150xc2x0 C.
Referring to FIG. 1D, after patterning the color filter 106A, the resulting structure is soaked in a developing solution. This soaking step erodes a portion of the surface of the metal line 107, leaving a high resistance oxide layer 108. This high resistance oxide layer 108 forms a poor contact with any metal ball connected thereto for driving the circuit.
It is, therefore, desirable to have a CMOS image sensor capable of preventing a surface of a metal line from being damaged, oxidized, or otherwise contaminated, thereby improving the performance of the semiconductor memory device.
In accordance with an aspect of the invention, there is provided a CMOS image sensor comprising: a semiconductor structure, wherein the semiconductor structure includes a unit pixel area and a pad area; a metal line formed on the pad area, wherein a portion of the metal line is exposed; a passivation layer formed on the unit pixel area and on the metal line such that the exposed portion of the metal line is left exposed; a planarized photoresist formed on a portion of the passivation layer; a micro-lens formed on a portion of the planarized photoresist; and an oxide layer formed on the micro-lens, the photoresist, and the passivation layer such that the exposed portion is left exposed.
In accordance with another aspect of the invention, there is provided a method for fabricating a CMOS image sensor, comprising the steps of: a) providing a semiconductor structure, wherein the semiconductor structure includes a metal line formed on an upper portion of the semiconductor structure; b) forming a passivation layer on the metal line; c) forming a planarized photoresist on a portion of the passivation layer; d) forming a micro-lens on a portion of the planarized photoresist; e) forming an oxide layer on the micro-lens, the photoresist, and the passivation layer; and f) forming a pad open mask and etching the oxide layer and the passivation layer to expose a portion of the metal line.